The invention relates to a computed tomography apparatus which includes a scanning unit which is rotatable relative to an examination zone, about an axis of rotation extending through the examination zone, and includes a radiation source for generating a radiation beam, a diaphragm arrangement which is arranged between the radiation source and the examination zone in order to form a fan beam traversing the examination zone from the radiation beam, and a two-dimensional detector arrangement which includes a plurality of detector elements and a part of the measuring surface of which detects primary radiation from the fan beam whereas another part of its measuring surface detects scattered radiation produced in the examination zone.
Detector elements that are not directly exposed to the primary radiation are struck by scattered radiation that arises along the relevant primary beam throughout the object to be examined. The momentum transfer, being proportional to the product of the energy of the scattered quanta and the sine of half the scatter angle, can then be reconstructed by means of an iterative algebraic reconstruction technique (ART) which is described in detail in the cited D1. For each voxel in the examination zone which is traversed by a primary beam the reconstruction results in a momentum transfer spectrum (the momentum transfer spectrum represents the intensity of the scattered radiation as a function of the momentum transfer) that is characteristic of the matter in the relevant voxel and hence provides information as regards the physical composition.
A drawback of such a method is addressed already in said D1 and consists in the comparatively long measuring time required for the acquisition of all measuring data necessary for the reconstruction. In the cited D1 it is indicated that this measuring time can be reduced by means of a computed tomography apparatus, for example an apparatus having a fan beam geometry, be it that the measuring data is then more strongly corrupted by undesirable scattered radiation. The method which is known from WO 99/45843 has this drawback. Each detector element can then receive scattered radiation from the entire scatter fan beam, that is, also scattered radiation that is scattered at a comparatively large scatter angle (so mainly Compton scattered radiation) and does not offer any information as regards the physical composition of the examination zone. Moreover, there is no reconstruction method enabling the reconstruction of the scatter density in the examination zone in such circumstances.
Therefore, it is an object of the present invention to construct a computed tomography apparatus in such a manner that it offers short acquisition times on the one hand and a perfect reconstruction of the momentum transfer spectrum in the examination zone on the other hand. On the basis of a computed tomography apparatus of the kind set forth this object is achieved according to the invention in that between the examination zone and the detector arrangement there is provided a collimator arrangement which includes a plurality of lamellas which are situated in planes that subdivide the fan beam into a number of segments so that the detector elements that are situated in a column extending parallel to the axis of rotation are struck by primary radiation or scattered radiation from the same segment. The collimator arrangement according to the invention has a dual function:
It prevents scattered radiation that has been scattered at a large scatter angle (so mainly Compton scattered radiation) from being incident on the detector elements.
It subdivides the fan beam into a number of segments which are to be considered approximately as a xe2x80x9cpencil beamxe2x80x9d, so that the momentum transfer spectra for the various voxels in the examination zone can be reconstructed by means of the method which is known from the cited D1.
In principle there are several possibilities for arranging the lamellas of the collimator arrangement that extend parallel to the axis of rotation or perpendicularly to the radiation beam.
One embodiment of the present invention enables the use of a computed tomography apparatus on the one hand for the determination of the momentum transfer spectra of the scattered radiation and on the other hand for the acquisition of the attenuation of the X-rays with an even further reduced acquisition time (cone beam CT) while using few additional means. In another embodiment of the present invention the necessary change-over takes place from a fan beam to a cone beam. Another embodiment deals with the processing of the various measuring data acquired in the various modes of operation.